This invention relates generally to radiographic systems for producing visual images of an irradiated subject, and is concerned more particularly with an X-ray radiographic system having automatic control means for ensuring that prescribed operating conditions have been met before permitting an X-ray exposure to be taken.
A radiographic system generally includes an X-ray generator disposed to direct an X-ray beam through a collimator and onto a suitably spaced image receptor. The X-ray generator usually comprises an X-ray tube enclosed within a shielded housing and having an evacuated envelope wherein an electron emitting cathode is disposed for directing an electron beam onto a focal spot area of an anode target. Generally, the focal spot area is maintained as small as possible in order to function as a point source for the resulting X-ray beam radiating therefrom. The X-ray beam passes through an X-ray transparent window in the tube envelope and through an aligned port in the housing. Accordingly, the collimator may be mounted over the port such that the X-ray beam passes through an adjustable aperture therein formed by a plurality of X-ray absorbent shutters. Thus, the collimated X-ray beam is provided with a suitable cross-sectional size and configuration for impinging on the image receptor.
One type of image receptor in general use comprises a rectangular film supported in a close-fitting cassette which may be installed in a wall-type holder, or in a tray located beneath the surface of a conventional X-ray table. A suitable collimator for providing the X-ray beam with a conforming rectangular cross section is disclosed, for example, in U.S. Pat. No. 3,581,094 granted to L. F. Peyser and assigned to the assignee of this invention. The collimator described therein comprises two orthogonally disposed pairs of opposing pivotal plates made of X-ray absorbent material for forming a rectangular aperture through which the X-ray beam passes. Each pair of opposing pivotal plates is operatively connected to a respective rotatable shaft for simultaneous and corresponding adjustment to define the associated dimension of the rectangular aperture, independently of the other dimension thereof. Thus, a patient may be positioned between the collimator and the image receptor such that the collimated X-ray beam passes through only a selected portion of the patient and produces an X-ray image thereof on the X-ray film.
Public health laws now require that positive means be provided for preventing generation of the X-ray beam until it is ensured that prescribed operating conditions have been satisfied and the incident cross-section of the X-ray beam conforms closely to the size of the receptor. After these conditions have been met, the incident cross section of the X-ray beam may be reduced to a size smaller than the size of the receptor. On the other hand, if the incident cross section of the X-ray beam is increased to a size larger than the receptor, it is required that generation of the X-ray beam be prevented in order to avoid exposing the patient to unnecessary X-radiation. Consequently, a number of prior art systems of the described type have been developed for complying with the requirements of the public health laws. However, these prior art systems generally are very costly and, consequently, are beyond the price range which general practitioners can afford.
Therefore, it is advantageous and desirable to provide a relatively simple and inexpensive radiographic system having means for ensuring that a patient will not be exposed to unnecessary X-radiation.